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<div class="moz-cite-prefix">Dear Charidimos,<br>
<br>
thanks very much for pointing to this, it is indeed an error in
the appendix of the tutorial. cfg.grid.pos should be based on the
subject-specific MRI or sourcemodel. So, correctly it should say
that you need to load sourcemodel.mat (i.e. subject specific grid)
and then use sourcemodel.pos(maxpowindx, :) <br>
The rationale in principle is that the warped grid has the same
size and is indexed the same as the original grid; that is why you
can use the index variable obtained from the MNI-warped source
reconstructed data. The virtual channel reconstruction should
however still be done in whatever coordinate system the subject's
anatomical data is in. I will update the appendix accordingly.<br>
<br>
Best,<br>
Jörn<br>
<br>
On 8/6/2013 8:24 PM, Charidimos Tzagarakis wrote:<br>
</div>
<blockquote
cite="mid:CAFN2X+fdvTW8jkx1EPS8C+J_FGRTQgTtYL38aYw7Gv6BbLtsxg@mail.gmail.com"
type="cite">Hi there, <br>
I have a question regarding virtual electrodes: How can I double
check that the MNI coordinates that I enter are correctly
interpreted?<br>
More specifically:<br>
I have followed the the extended beamforming tutorial in the
"Appendix" and have been able to transpose it to my data (I have
4D/BTi MEG files and Dicom mri volumes).The part I am not certain
about is the call to the LCMV beamformer.<br>
Here it is in my case ( I just want to get the voxel were the
power is max in the previous analysis):<br>
cfg = [];<br>
cfg.method = 'lcmv';<br>
cfg.vol = volfcm;<br>
cfg.grid.pos = source_diff.pos(maxpowindx, :);<br>
cfg.grid.inside = 1:size(cfg.grid.pos, 1);<br>
cfg.grid.outside = [];<br>
cfg.grad=senscm<br>
source_idx = ft_sourceanalysis(cfg, tlock);<br>
My headmodel does not include MEG channel information so I need to
also define the cfg.grad parameter. What I don't understand is how
cfg.grid.pos (which is in MNI coordinates as per the previous part
of the tutorial) can be correctly applied to the coordinates of
the headmodel and the channels since these 2 are not in MNI
coordinates (in my case they are in 4d/Bti coordinates and
rescaled to cm).In the previous part of the tutorial this is (if I
understand correctly) accomplished because the leadfield used when
calling ft_sourceanalysis was created using and MNI-warped
template. I don't see where the equivalent part is when estimating
the virtual electrode though.<br>
Any advice you may have would be much appreciated.<br>
Best,<br>
Haris<br>
<br>
<br>
Charidimos [Haris] Tzagarakis MD, PhD, MRCPsych
<br>
University of Minnesota Dept of Neuroscience and Brain Sciences
Center
<br>
<br>
<br>
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<br>
<br>
<pre class="moz-signature" cols="72">--
Jörn M. Horschig
PhD Student
Donders Institute for Brain, Cognition and Behaviour
Centre for Cognitive Neuroimaging
Radboud University Nijmegen
Neuronal Oscillations Group
FieldTrip Development Team
P.O. Box 9101
NL-6500 HB Nijmegen
The Netherlands
Contact:
E-Mail: <a class="moz-txt-link-abbreviated" href="mailto:jm.horschig@donders.ru.nl">jm.horschig@donders.ru.nl</a>
Tel: +31-(0)24-36-68493
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Visiting address:
Trigon, room 2.30
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NL-6525 EN Nijmegen
The Netherlands</pre>
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